https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/connection/11x/design/guide/b_11xcucdg/ b_11xcucdg_chapter_00.html#ID-2342-00000133

A Cisco IP Phone 8800 series phone registered using Mobile and Remote Access (MRA) ultimately registers to the Cisco Unified Communications Manager (UCM). MRA provides secure remote connectivity for endpoints, but it does not act as the endpoint registration platform itself.
Cisco UCM is the central call control system that manages endpoint registrations, call routing, and feature provisioning for Cisco IP Phones. Even with MRA, the actual phone registration happens with Cisco UCM.
Cisco Expressway-C and Expressway-E are used to enable secure MRA. However, they do not directly register the endpoints.
Cisco Unified Presence is used for presence and instant messaging services, not for IP phone registration.

You choose your Codec Preference List in the Region configuration when creating new relationships between Regions.

The Advanced Ad Hoc Conference Enabled service parameter allows a Cisco UCM user to add additional participants directly to an existing point-to-point call, escalating it into an ad hoc conference, without needing to first place the original call on hold and start a new one. This capability applies to both audio and video calls when a conference bridge with sufficient capabilities is available.